The world’s water is a finite and precious resource, constantly under industrial, agricultural, and domestic pressure. Water scarcity concerns have soared in recent years as more countries around the world have implemented more stringent regulations to meet their demands. Water scarcity coupled with rapid population growth in some of these countries has brought increased attention to appropriate water use and disposal practices. According to the [58] report, water demand in the last decade has increased by more than 100%, and projections show that it is expected to increase in countries with developing economies. Industrial and agricultural processing industries have played a vital part in the economic development of these countries in past years, and with large-scale development and population growth, it is also expected to increase in the coming years [57]. Water quality concerns have been raised by international and governmental bodies around the world largely due to the accumulation of organic and inorganic suspended matter and nitrate as well as soluble phosphorus in the natural water bodies [26]. Different studies have shown that only a few industries in developing regions have their wastewater treated before disposal into the aquatic environment [17, 58]. Many of these contain chemicals that pose a risk to human health and have often been associated with loss of biodiversity and ecological damage. Some of them are persistent, toxic, and partly biodegradable and hence are not easily removed by conventional wastewater treatment plants [2, 44]. With recent environmental pollution problems, there is therefore a need to monitor, control, and develop sustainable and economically efficient methods for the treatment of industrial wastewater before its disposal into the environment [1, 52].
The brewery industry is water-intensive as a large volume of water is required for the daily production of beer. For instance, an average of 6.0 hl is required to produce 1 hl of clear beer. In the brewing process, water not only serves as the main ingredient of the beer but is also used in steam raising, cooling, washing of floors, packaging, and cleaning during and after each batch operation [18]. The wastewater effluents from the brewery process also contribute to soil pollution in the cases of inappropriate treatment and land discharge [43]; it is also reported to cause inhibition of seed germination, reduction of soil alkalinity, loss of soil manganese, and damage in agricultural products [1, 52].
A wide range of conventional and advanced methods have been adopted for the treatment of brewery wastewater [52, 56]. Some of the conventional treatment methods include anaerobic treatment with the recovery of biogas, followed by aerobic treatment. However, some phenolic compounds have been reported in this type of wastewater [8, 25, 53] which are not readily biodegradable and therefore cannot be removed by conventional methods. Some other effective technologies have also been proposed for brewery wastewater treatment; however, for such technologies to be implemented, it would require environmental regulations which can be costly and relatively complex [27, 52].
Advanced oxidation treatment processes have also been widely used in the treatment of distillery and brewery wastewater; they operate through the generation of hydroxyl radicals and other oxidant species to degrade organic compounds in wastewater [48]. Advanced oxidation process (AOPs) technologies can also be applied by a combination of hydrogen peroxide/ultraviolet irradiation (H2O2/UV), ozone/ultraviolet irradiation (O2/UV), and ozone/hydrogen peroxide (O2/H2O2), and ozone and hydroxyl radicals \(\Big({}{}^{\bullet } OH\Big)\), which are robust oxidants capable of oxidizing a wide range of organic compounds when dissolved in water [3, 10]. Several technologies have been successfully used in the removal of highly complex molecules that are bio-refractory in nature [9, 23, 29, 32]. AOPs offer an attractive approach owing to their high oxidation potential and hydroxyl radicals produced, which helps in the degradation and mineralization of pollutants [11, 21, 47].
Fenton’s oxidation process is a well-known AOPs based on the Fenton reaction. The Fenton process is a catalytic cycle of reaction between iron (Fe2+) and hydrogen peroxide (H2O2) to produce hydroxyl radicals. Fenton oxidation technology produces hydroxyl radicals (OH), with the reaction generally occurring in an acidic medium between pH 2 and 4 [37, 49] resulting in the precipitate formation and de-colorization of effluent [7]. Fenton technology produces a homogeneous reaction that is ecologically friendly. The efficiency of the Fenton reaction depends mainly on (H2O2) concentration, the Fe2+/H2O2 ratio [31], pH, and reaction time. In addition, the initial concentration of the pollutants and their character, as well as temperature, has a substantial influence on final efficiency [45, 46]. Fenton’s reagent is characterized by its cost-effectiveness, simplicity, and suitability for treating aqueous wastes with variable compositions [3, 9]. The Fenton pro cess involves the application of Fe2+ and H2O2 for the production of hydroxyl radicals. Ferrous ion is oxidized by H2O2 to ferric ion to hydroxyl radical, and a hydroxyl anion [9]. The reaction is shown in the following steps.
$${Fe}^{2+}+{H}_2{O}_2\to {}{}^{\bullet } OH+{OH}^{-}+{Fe}^{3+}$$
(1)
$${Fe}^{3+}+{H}_2{O}_2\to {Fe}^{2+}+{H}^{+}+{HOO}^{\bullet }$$
(2)
$${Fe}^{3+}+{HOO}^{\bullet}\to {Fe}^{2+}+{H}^{+}+{O}_2$$
(3)
$${Fe}^{2+}+{}{}^{\bullet } OH\to {Fe}^{3+}+{OH}^{-}$$
(4)
$${}{}^{\bullet } OH+{H}_2{O}_2\to {H}_2O+{HOO}^{\bullet }$$
(5)
$${Fe}^{2+}+{HOO}^{\bullet}\to {HOO}^{-}+{Fe}^{3+}$$
(6)
$${}{}^{\bullet } OH+{}{}^{\bullet } OH\to {H}_2{O}_2$$
(7)
$${}{}^{\bullet } OH+ organics\to products+{CO}_2+{H}_2O$$
(8)
Ferric ion is reduced back (typically in the presence of irradiations) to ferrous ion, a peroxide radical, and a proton by the same H2O2 [37, 49]. The rate of reaction (1) is around 63 M−1 s−1, while the rate of reaction (2) is about 0.01–0.02 M−1 s−1 (Kang et al. 2002 [36, 48, 51];). This shows that the ferrous ions are consumed faster than they are being generated. The hydroxyl radicals then degrade the organic compounds in reaction (8), and H2O2 also reacts with Fe3+ via reaction (2) [20]. Fenton chemistry has been studied by many researchers for oxidation of different organic pollutants, including aromatic and phenolic compounds, pesticides, herbicides, and organic dyes [7, 12, 28, 30, 35, 36, 54, 59].
While the Fenton process has recorded success on a laboratory scale, the process still finds lesser application on an industrial scale largely due to its ineffectiveness in reducing certain refractory pollutants, such as acetic acid, acetone, carbon tetrachloride, methylene chloride, n-paraffins, maleic acid, malonic acid, oxalic acid, and trichloro-ethane, and also due to the high amount of total dissolved solids generated during the process [48, 51]. To manage and improve the quality of sludge generated using the Fenton reagent, the electro-Fenton (EF) process was developed from the principle of ionization, oxidation, and separation of wastewater constituents at the atomic level using electric currents [41]. The development of the EF process solved the problem of imbalance in the Fe/H2O2 ratio (which leads to a lower rate of oxidation) and extensive use of oxidants (such as H2O2) in the Fenton’s process [16].
The EF process reduces the quality of sludge generated by recycling the ferric back to ferrous salt electrochemically. The converted ferrous salt then participates in the oxidation process again, this recycling process can be done in two ways: (1) in situ recycling and (2) ex situ recycling. In in situ recycling, the quantity of the added ferrous catalyst is lower compared to the conventional Fenton process. During this process, the ferric salt formed after the Fenton process is converted back into ferrous salt at the cathode [6, 15]. This reaction process is also called the Fered Fenton process. The reaction taking place at the anode is:
$${H}_2{O}_2\to 2{H}^{+}+1\left/ 2\ \right.{O}_2+2{e}^{-}$$
(9)
while the reaction taking place at the cathode is:
$${Fe}^{3+}+{e}^{-}\to {Fe}^{2+}$$
(10)
The main advantage of the Fered Fenton process is the molar ratio of Fe ions and H2O2 available at any time in the reactor, which can be easily controlled and maintained at the optimal level; with this, the Fered Fenton process not only reduces the quantity of ferrous salt produced, but it also accelerates and increases the degradation of organic compounds compared to the conventional Fenton process [23, 29, 50, 60].
For the ex situ recycling process, the sludge recycling process takes place in the same reactor as the in situ recycling process. However, the sludge after the neutralization stage is acidified to the required pH and then passed through an electrochemical cell where the ferric is converted back to ferrous salt [48, 51].
However, despite the potential the EF process presents, there is still a need to seriously assess high-cost features such as the need to add and maintain appropriate ionic Fe concentrations, reaction time, the concentration of oxidant, and the requirement to remove the iron species and neutralize the acid of the aqueous effluent after treatment [14, 40].
The objective of this study is to analytically examine the influence and interaction of oxidant (H2O2) and reaction time on biological oxygen demand (BOD) and chemical oxygen demand (COD) removal efficiency during the EF process. The study looks into the overall removal efficiency of the EF process (by adding iron from external sources) on the treatment of brewery wastewater. The study also characterizes the interaction between the concentration of oxidant and reaction time during the EF process in a fully submerged electrolytic cell. The treatment of high-strength brewery wastewater can be further developed from the results this study presents.
